Fail-safe pulsating peak detection circuit

ABSTRACT

This disclosure relates to a fail-safe peak detector for detecting the presence of pulsating input signals and for providing an output signal which is proportional to the peak values of the input signals. The detector includes an amplifier having its input transformer coupled to the input signals and having its output transformer coupled to a bridge rectifier which is coupled to the coil of an electromagnetic relay which in turn controls an electrical contact of an indicating circuit.

0 United States Patent 1151 3,660,731 Darrow 1 May 2, 1972 541 FAIL-SAFEPULSATING PEAK 2,874,339 2/1959 Perlman ..317/148.5 DETECTION CIRCUIT2,934,637 4/1960 Wilcox... .....317/148.5 X

3,209,212 9196 '11' ..31 14 [72] Inventor: John 0. G. Darrow,Murrysville, Pa. 5 Bl 7/ 8 5 X [73] Assignee: Westinghouse Air BrakeCompany, Swiss- Primary Examiner-L. T. Hix

vale, Pa. Attorney-H. A. Williamson, A. G. Williamson, Jr. and J. B. 22Filed: Feb. 18, 1970 smak [2]] Appl. No.: 12,297 [57] ABSTRACT Thisdisclosure relates to a fail-safe peak detector for detect- U-S- R, 3 l6 the presence of pulsating input ignals and for providing an [51] Int.Cl. ..HOlh 47/32 output Signal which is proportional to the peak valuesof the [58] FIG of Search ..3l7/l47, l48.5, DIG. 6, 138 input i l Thedetector includes an amplifier having its input transformer coupled tothe input signals and having its [56] References cued output transformercoupled to a bridge rectifier which is cou- UNITED STATES PATENTS pledto the coil of an electromagnetic relay which in turn controls anelectrical contact of an indicating circuit. 2,695,378 11/1954 Irvin..317/DIG. 6 3,252,141 5/1966 Galin ..317/138 X 15 Claims, 1 DrawingFigure To Trudi 1 FAIL-SAFE PULSATING PEAK DETECTION CIRCUIT Myinvention relates to a fail-safe current-type peak detector and moreparticularly to an electronic receiver detection circuit for detectingpulsating input signals and for producing an output signal which isindependent of the pulse length of the pulsating input signals.

In railroad classification yards relatively short car detection trackcircuits are employed at certain locations, such as track switches, todetect the presence of a car or cut of cars within a given tracksection. These circuits incorporate a transmitter and a receiverelectrically coupled across the rails at preselected spaced points alongthe track. A common deterrent to the successful use of track circuits inclassification yards has been the fact that surface films are formed onthe rails. These films are generally the result of dirt, rust, or greaseas well as other contaminations and foreign materials which areordinarily present in a railroad millieu. The poor condition of railsrequires that a high power type of transmitter be employed in trackcircuits for classification yards so that effective film penetration andshunting may take place when the wheels of the car approach thereceiver. In order to reduce the power requirements of the track circuitequipment, it has been found advantageous to operate the transmitter onan intermittent rather than on a continuous bases. This is accomplishedby limiting the duty cycle of the transmitter. That is, the transmitteris not ON at all times but is arranged to generate short bursts orpulses of high frequency voltage signals having sufficient amplitude topenetrate the rail film. Thus, it will be appreciated that by selectinga satisfactory duty cycle and an appropriate pulse repetition rate, theaverage amount of power required by the transmitter is appreciablyreduced. However, a need for improving the receiver signal detectioncircuit has arisen in pulsating types of high frequency track circuits.It has been found that previous types of receivers and particularlytheir detector circuits are not completely acceptaparticularly theirdetector circuits are not completely acceptable in that an adversecondition arises when th e f QNftime of time operation f l urther, ithas been found that ordinary capacitive peak detectors require largepower sources which defeat the use of a pulsing transmitter in the trackcircuit. In addition, a capacitive peak detector generates large currentsurges and transients which have an adverse effect on active elements,such as transistors and cause erratic operation of other relatedcircuits. Thus, in the past, averaging types of voltage detectioncircuits were commonly employed in the receiver portion of the highfrequency track circuits. It will be appreciated that the average amountof voltage is directly proportional to the duty cycle of thetransmitter. Thus, as the duty cycle or ON time of the transmitterincreases so does the average amount of output voltage of the receiverincrease. This, of course, creates a situation where the vehicleshunting at the receiver end changes so that the track circuit iseffectively shortened. That is, the track circuit length is decreasedsince the point where the approaching car wheels and axle shunt thetransmitter signals is significantly closer to the receiver. Thus, theindication relay remains picked up longer so that the car is effectivelyallowed to move further into the track section than had originally beenplanned. Such receiver operation is intolerable in classification yardssince any shortening of the track circuit length can result in corneringand switch splitting by the moving of railway cars. A further requisiteof vital types of high frequency track circuits is that the apparatusoperates in a fail-safe manner. For example, it is of paramountimportance to ensure that a circuit or component failure must beincapable of simulating a safe condition. Such operation is mandatory inorder to prevent injury to attending personnel and to eliminate damageto the railroad equipment and lading.

Accordingly, it is an object of my invention to provide a failsafeelectronic peak detection circuit.

Another object of my invention is to provide an improved peak pulsedetector which is independent of the pulse width.

A further object of my invention is to provide an improved peakdetection circuit having an output current which is inde pendent of theduty cycle of the input voltage.

Yet another object of my invention is to provide an improved trackcircuit receiver for detecting the peaks of pulsating signalstransmitted through the rails by a high power transmitter having a lowduty cycle.

Still another object of my invention is to provide a fail-safe detectorhaving an output current which is unrelated to the average amount ofinput voltage.

Still a further object of my invention is to provide a pulsating peakdetection circuit which is unafiected by an increase in the duty cycleof the input voltage, which is free of current surges and spikes andwhich is incapable of producing an output signal during the absence ofthe input voltage.

Still yet another object of my invention is to provide an improvedpulsating current-type peak detector which operates in a fail-safemanner so that a circuit or component failure is incapable of simulatinga safe condition.

Yet a further object of my invention is to provide an im provedfail-safe type of peak detection circuit having a voltage-to-currentconverter transformer coupled to a source of input voltage and alsotransformer coupled through AC-to-DC converter to an electromagneticrelay.

Still yet a further object of my invention is to provide a new andimproved peak detector which is economical in cost, simple inconstruction, reliable in operation, durable in use and efficient inservice.

Other features, objects and advantages of my invention will appear asthe specification progresses.

In the attainment of the foregoing objects, my fail-safe pulsating peakdetection circuit includes a voltage-to-current converter comprising aclass B push-pull amplifier. The amplifier includes a pair of NPNtransistors each having an emitter, a collector and a base electrode. Aninput transformer having a primary and a pair of secondary windingssupply AC input signals to the amplifier. The primary winding isconnected to a source of AC pulsating input signals having a relativelyshort duty cycle while the secondary windings each have one end directlyconnected to the base electrode of a respective transistor and have theother end connected to ground. An output transformer having a pair ofprimary windings and a secondary winding couples amplified AC signals toa diode bridge rectifier. One end of each primary winding is connectedto a DC supply terminal while the other end is connected to thecollector electrodes of the respective transistors. The emitterelectrodes of the transistors are commonly connected to ground throughan emitter load resistor. The secondary winding is connected to the ACtenninals of the bridge rectifier while the DC terminals of the bridgerectifier are connected to the coil of an electromagnetic relay. Thecoil includes a series inductance and resistance, the parameters ofwhich are selected to provide a long time constant relative to the pulserepetition period. Thus, the coil of the electromagnetic relay has acurrent flowing through it which is proportional to the peak values ofthe AC input signals and its self-induction causes load current tocontinue to flow through it during the interval period between thesignal pulses.

For a more complete understanding of my invention as well as realizingother objects and advantages thereof, reference is made to the followingdetail description taken in conjunction with the accompanying FlGURE,which is a schematic diagram of the pulsating peak detection circuit inaccordance with my invention.

Referring to the single FIGURE of the drawing, there is shown a voltagepeak detection circuit which is generally characterized by thenumeral 1. The detection circuit 1 forms the receiver portion of thehigh frequency track circuit and provides an indication of the presenceand absence of a railway car within the track circuit. The receiverdetection circuit is normally coupled to the opposite rails at apreselected distance from the point where the transmitter is coupled tothe track rails. Thus, the high frequency voltage signals generated bythe transmitter are transmitted through the rails and are picked up fromthe rail and applied to the input terminals R1 and R2.

The transmitter may take the form of an ordinary AC signal oscillatorhaving a frequency of 50 kilohertz. The transmitter is pulsed 120 timesper second and preferably has a duty cycle of approximately 8 topercent. As previously mentioned, the purpose of utilizing anintermittent or low duty cycle rather than a continuous or full timetransmitter is one of economics. Thus, the power requirements andoverall cost of the track circuit equipment may be appreciably reducedsince less costly components and supply sources may be employed.However, it will be appreciated that the peak values of the voltagesignals produced by the low duty cycle transmitter must be of sufficientmagnitude for penetrating rail films.

Returning now to the receiver end of the track circuit, it will beunderstood that the pulsating voltage signals are picked up from therails and conveyed by suitable conductors to the input terminals R1 andR2 of the primary winding P1 of transformer T1. The transformer T1includes a pair of center tapped secondary windings S1 and S2 formingthe input circuit of the voltage-to-current converter. Thevoltage-to-current converter takes the form of a push-pull amplifieroperating in class B service. The push-pull amplifier includes a firstPNP-transistor Ql having emitter e1, a collector electrode 01 and a baseelectrode b1 and a second PNP-transistor Q2 having emitter electrode e2,a collector electrode 02, and the base electrode b2. The upper terminalon the secondary winding S1 is directly coupled to the base electrode b1of transistor Q1 while the lower terminal of secondary S2 is directlyconnected to the base electrode b2 of the transistor Q2. The center tapof the secondary windings is connected to one end of the carboncomposition type of emitter degenerating resistor R1. The one end ofresistor R and center tap are connected to a point of reference, suchas, ground. The other end of resistor R is connected to a junction pointwhich is common to both emitter el and e2 of transistors Q1 and Q2,respectively. The output circuit of the push-pull amplifier includes apair of primaries P2 and P3 of a transformer T2. As shown, the upperterminal of the primary winding P2 is directly connected to thecollector electrode cl of transistor Q1 while the lower terminal ofprimary winding P3 is directly connected to collector electrode c2 oftransistor Q2. The center tap of the primary windings is connected tothe positive potential +V of a suitable source of DC supply voltage (notshown). The transformer T2 includes a secondary winding S3 which isdirectly connected to the AC terminals of a full-wave bridge rectifierB. As usual, the bridge rectifier B includes a plurality of diodes D1,D2, D3 and D4 which rectify the AC signals into a DC output. As shown,the positive terminal of the bridge rectifier B is directly connected tothe coil or winding W of a vital type of electromagnetic relay whichcomprises an inductance LR and a given amount of series resistance RR.The other end of the relay coil or winding W is directly connected tothe negative terminal of the bridge rectifier B. The coil W operates amovable contact a which is normally closed when the relay is energizedthereby indicating the absence of a car from the track circuit. The timeconstant LR/RR of the series inductance-resistance circuit of the coilis chosen to be relatively large when compared to the dead time or tothe interval between the signal pulses, the purpose of which we describehereinafter in greater detail.

It will be noted that the presence of the resistor R ensures that theinput circuit of the push-pull amplifier is essentially a voltage sourcewhile the output circuit of the push-pull amplifier is essentially acurrent source. Assuming negligible voltage drop across the variouselectrodes of the transistors, it will be noted that the voltage dropacross resistor R is effectively equal to the input voltage whichappears across the secondary windings S1 and S2 of the input circuit.Further, the amount of output current flowing through the collector oftransistors Q1 and Q2 is effectively equal to the current flowingthrough the emitter of the transistors which in turn flows through theresistor R. The turns ratio of the transfonner T2 is so selected that ahigh output impedance is presented by the diode bridge when compared tothe impedance of the relay coil. Thus by selecting appropriate turnsratio and resistance parameter, the output current rather than theoutput voltage becomes a function of the peak input voltage.

Proceeding now with the detailed description of the operation of thepulsating peak detection circuit according to the present understandingof the invention, it is initially assumed that no railway car is withinthe limits defined by the track circuit section. Further, assumingconditions are normal and that the equipment is functioning properly,the pulsating voltage signals generated by the transmitter are appliedover suitable conductors to the track rails. These pulsating voltagesignals are carried by the rails and appear at the receiver end of thetrack circuit. The pulsating voltage signals are picked up from therails and are applied to the input terminals R1 and R2 of the primarywinding P1 of transformer T1. it is presumed for the purpose ofconvenience that a positive alternation of the 50 ltilohertz signal isinitially received so that the terminal R1 is positive with respect toterminal R2.

The transformer T1 is wound such that primary winding P1 induces avoltage having a polarity which in this instance causes the upperterminal of secondary winding S1 to be positive and the lower terminalof secondary winding S2 to be negative. Thus, the positive swing in thesecondary winding S1 forwardly biases the base-emitter electrodes bl-elso that transistor Q1 is rendered conductive while the positive swing inthe secondary winding S2 opposes and reversely biases the base-emitterelectrodes b2-e2 so transistor Q2 remains nonconductive. Thus, outputcurrent flows from the positive terminal +V, through the primary windingP2 of transformer T2, through the collector c1 and emitter e1 oftransistor O1 to junction J1 through resistor R to ground. Let us assumethat the transfonner T2 is wound in phase so that the polarity of thesignal induced in secondary winding S3 at this time is the same as thatof the primary winding P2. Thus, current flows from the lower terminalof secondary winding S3, through diode D1, through the inductance LR andresistor RR of the relay coil, through diode D2 to the upper terminal ofthe secondary winding S3. Thus, with current flowing through coil W, theelectromagnetic relay will become energized thereby closing contact a.The closing of contact a completes the indication circuit which, inturn, designates that no vehicle or car is present in the track circuit.Now when the negative alternation of the 50 kilohertz signal appearsacross terminals R1 and R2 the primary winding P1 induces a signal insecondary windings S1 and S2 which is in phase therewith. Now theinduced voltage in secondary winding S2 will forwardly bias thebase-emitter electrodes b2-e2 so that transistor Q2 will conduct whilethe induced voltage in secondary winding S1 will reversely bias thebase-emitter electrodes bl-el so that transistor Ql will becomenonconductive. Thus, current now flows from the DC source terminal +Vthrough the primary winding P3, through the collector c2 and emitter e2of transistor Q2, to the junction J 1, through resistor R to ground.Now, the primary winding P3 induces a signal in secondary winding 83which is of the opposite polarity to that induced by primary winding P3,thus current will now flow from the upper terminal of secondary windingS3, through diode D3, through inductance LR and resistor RR, of therelay coil, through diode D4 to the lower negative terminal of thesecondary winding S3. During the next half cycle, the transistor Q1 willagain conduct and produce a signal across primary winding P2. Thisalternate operation of the transistor will continue for the entire dutycycle of the transmitter so that the corresponding signals are generatedin the respective halves of the primary windings of transformer T2.Thus, current will continue to be supplied to the coil W of theelectromagnetic relay for the entire pulse width or duty cycle.

With a zero-biased class B push-pull amplifier a slight amount ofcrossover distortion occurs, but by and large the waveform of the outputcurrent closely resembles the curve of the input voltage which in thiscase may be sine-wave. As

previously mentioned, the duty cycle or pulse width of the transmitteris only approximately percent of the total time or pulse repetitionperiod. it will be appreciated that in order to preclude an erroneouscar presence indication, the front contact a of the electromagneticrelay must remain closed during period or time in which the pulsating S0kilohertz signals do not appear on input terminals R1 and R2. During thetransmitter off-time period, the characteristics of the relay, namely,the inductive and magnetic properties are employed for maintaining therelay in its picked-up condition. As previously mentioned, the timeconstant LR/RR of the relay coil is chosen to be relatively large incomparison to the offtime period of the transmitter. The drasticreduction, namely, the cutting off of current being supplied by theamplifier causes a variation in the magnetic field which results in thegeneration of a back EMF. Thus, the self-induction of the inductor LRcauses current to continue to flow in the same direction through theseries inductance circuit. it will be noted that the polarity of thediodes in bridge rectifier B is such that effective snubbing or shuntingaction will occur, and therefore sufficient current flows through thecoil W for holding contact a closed.

Hence when the next succeeding 50 kilohertz signal pulse appears acrossterminals RI and R2 current will again be supplied to theelectromagnetic coil W by the push-pull amplifier. Thus, it will beappreciated that the output current is purely a function of the peakvalue of the input signal pulses due to the integration of the RLcircuit.

Now, let us assume that for some unforeseen reason the duty cycle of thetransmitter increases. It will be noted that the length of the dutycyclehas no effect on the operation of the presently described detectioncircuit. That is, an increase in the transmitter duty cycle even to afull ON" condition does not change the amount of output current flowingthrough the electromagnetic coil W. Since the peak values of the 50kilohertz signals are independent of the pulse width, it is impossibleto increase output current level of the pulsating peak detectioncircuit 1. Such operation ensures that the opening of the relay contacta will coincide with the passing of each railway vehicle or car at apreselected point in the track circuit. For example, when the railwaycar approaches a certain point at the receiver end of the track circuit,the 50 kilohertz pulsating voltage signal being produced by thetransmitter will be effectively shunted by the front wheels and axle.With no voltage signal available on terminals R1 and R2 the push-pullamplifier returns to a quiescent condition and after a short period oftime which is dependent upon the decay period of the series inductancecircuit the contact a will open. Thus the opening of contact a willaccurately signal the presence of a railway vehicle within the definedlimits of the track circuit.

Further it will be noted that the presently described peak detectioncircuit operates in a fail-safe manner in that no circuit or componentfailure is capable of simulating a false condition. That is, theelectromagnetic relay is incapable of being energized when any failureoccurs in the detection circuits. For example, if any element of thecircuit, with exception of resistor R becomes short-circuited the outputpower to the electromagnetic relay is either removed or drasticallyreduced so that contact a becomes opened to positively signify thepresence of a railway car in the track circuit. It will be noted thatthe shorting of any transformer winding reduces the transformer ratiowhich in turn diminishes the amount of current flowing through coil W.Similarily, any short circuiting of the transistor electrodes destroysthe amplifying characteristics of the amplifier, which, in turn reducesthe output current. A short-circuited diode results in a shunting actionin the relay coils so that little, if any, output current is availablefor energizing the relay. As is previously mentioned, the resistor R ispreferably constructed of carbon composition so that it is improbable,if not impossible, for the resistor to short circuit. It will be furthernoted that if any circuit component becomes opened either thetransformer coupling or the amplifying characteristics of this detectioncircuit are destroyed so that it is impossible to increase current flowthrough the relay coil W. For example, an open-circuited primary orsecondary winding either completely eliminates the signal or effectivelyreduces the signal to one-half of its peak value so that no current orat least less current will flow through the coil W of the relay. Anopen-circuited transistor destroys the amplifying characteristics of apush-pull amplifier so that no increase in current can result. Theopening of resistor R completely removes the input signal from thepush-pull amplifier so that current completely ceases to flow throughthe coil W of the relay. Similarly, opening of any one or more of thediodes causes at least one-half of the cycle, if not both alternationsto disappear so that the output current is either reduced or removed.Thus, a failure in the presently described pulse peak detection circuitis incapable of falsely energizing the relay.

Thus, the pulsating peak detector circuit 1 not only is unaffected by anincrease in the pulse width of the pulsating input signal which could becaused by a circuit failure in the transmitter, but also is incapable ofproducing a false output due to the presence of a circuit or componentfailure.

While my invention has been described with reference to track circuitsfor classification yards, it should be understood that the detectioncircuit may be used in other track circuit applications. It is alsounderstood that the presently described detection circuit may be usednot only in other railroad applications but also in other industrial,commercial as well as other places where similar needs and conditionsexist.

Although NPN transistors have been illustrated, it is understood thattransistors of the opposite conductivity, that is, PNP transistors maybe used in the circuit with a reversal of energizing potential as iswell known.

Further while my invention has been described with reference to aparticular embodiment thereof, it will be understood that variousmodifications, changes and variations may be made by those skilled inthe art without departing from the invention. The appended claims aretherefore intended to cover all such modifications within the truespirit and scope of the invention.

Having thus described my invention, what I claim is:

1. A fail-safe peak detector for detecting a pulsating input signal andfor producing an output signal which is independent of the pulse widthof the pulsating input signal comprising, a source of coded pulsatinginput signals havinga low duty cycle in comparison to the pulserepetition rate, a first coupling circuit connected to said source ofcoded pulsating input signals, a voltage-to-current converter having itsinput connected to said first coupling circuit, an AC-to-DC converterconnected to the output of said voltage-to-currcnt converter by a secondcoupling circuit, and an integrating circuit means coupled to the outputof said AC-to-DC converter for causing load current to continue to flowduring the interval between successive coded input pulses and said peakdetector being incapable of increasing said load current during acritical circuit or component failure.

2. A fail-safe peak detector as defined in claim 1, wherein said firstcoupling means comprises a transformer having a primary and a pair ofsecondary windings.

3. A fail-safe peak detector as defined in claim 1, wherein saidvoltage-to-current converter comprises an amplifying circuit having twoactive elements.

4. A fail-safe peak detector as defined in claim 1, wherein saidvoltage-to-current converter comprises a class B pushpull amplifier.

5. A fail-safe peak detector as defined in claim 1, wherein saidAC-to-DC converter comprises a full-wave bridge rectifier.

6. A fail-safe peak detector as defined in claim 1, wherein said secondcoupling circuit comprises a transformer having a pair of primarywindings and a secondary winding.

7. A fail-safe peak detector as defined in claim 1, wherein saidintegrating circuit means includes an electromagnetic relay.

8 A fail-safe peak detector as defined in claim 7, wherein saidelectromagnetic relay includes a magnetic coil coupled to the output ofsaid AC-to-DC converter and an electrical contact connected to anindication circuit.

9. A fail-safe peak detector as defined in claim 8, wherein saidmagnetic coil includes a series inductance and resistance elements.

10. A fail-safe peak detector as defined in claim 9, wherein the timeconstant of said series inductance and resistance elements issubstantially longer than the pulse repetition period of said codedpulsating input signal.

11. A fail-safe current-type of peak detection circuit comprising asource of coded pulsating signals having a low duty cycle in comparisonwith the pulse repetition rate, an input transformer having a primarywinding electrically connected to said pulsating signal source andhaving a pair of secondary windings, a push-pull amplifier, said pair ofsecondary windings electrically connected to the inputs of saidpush-pull amplifier, and an output transformer having a pair of primarywindings and a secondary winding, said pair of secondary windingselectrically connected to the output of said push-pull amplifier, arectifier having its input terminals connected to said secondary windingand having its output terminals connected to a coil of anelectromagnetic relay which remains picked up during the off periods ofsaid coded pulsating signals and which is incapable of being picked upby a critical circuit or component failure.

12. A fail-safe current-type of peak detection circuit as defined inclaim 11, wherein said coded pulsating signal source comprises a highfrequency signal source having a duty cycle of approximately 10 percentof the time.

13. A fail-safe current-type of peak detection amplifier includes a pairof semiconductive devices operating in class B service.

14. A fail-safe current-type of peak detection circuit as defined inclaim 11, wherein said rectifier comprises a fullwave bridge rectifier.

15. A fail-safe current-type of peak detection circuit as defined inclaim 11, said coil of said electromagnetic relay comprises a seriesconductive resistive circuit having a time constant which is relativelylarge in comparison with said pulse repetition rate.

' I UNITED STATES PATENT OFFICE, 1 CERTIFICATE OF CORRECTION rum No. 366 Q ,73] me a "May 2, 1972 g John 0., G Darrow :It 1: certified thaterror appears 1n thei above-identified patent and that id Letters Patentare'hereby corrected as shown below:

I" p p I Claim 13-, Colu'nin' 8 should read 'I' Av fail-safe current--type' 'of peak detection circuit :as

defined in Claim 11, wherein said push-pull amplifier includes a pairof; semiconductive devices operating inclass B service." I

finstead of I '"Arfail-s'afe current-type of peak detection amplifierincludes a pair of semiconductive' devices operating in class B.servicefl I Signed and sealed this 17th day of October 1972.

(SEAL) 'Attest: I

I ROBERT GOTTSCHALK EDWARD M.FLETCHER,JRW. Q Attesting OfficerCommissioner of Patents 233 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent 3.660.73l Dated May 2, 1972 Inventor) John O. G.Darrow It in certified that: error appears in the. above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Claim 13, columns should read "Av fail-safe current-type 'of peakdetection circuit as defined in Claim 11, wherein said push-pullamplifier includes a pair of semiconductive devices operating in class Bservice."

instead of "A fail-safe current-type of peak detection amplifierincludes a pair of semiconductive devices operating in class B service."

Signed and sealed this 17th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTI'SCHALK Attesting Officer Commissionerof Patents

1. A fail-safe peak detector for detecting a pulsating input signal andfor producing an output signal which is independent of the pulse widthof the pulsating input signal comprising, a source of coded pulsatinginput signals having a low duty cycle in comparison to the pulserepetition rate, a first coupling circuit connected to said source ofcoded pulsating input signals, a voltage-to-current converter having itsinput connected to said first coupling circuit, an AC-to-DC converterconnected to the output of said voltage-to-current converter by a secondcoupling circuit, and an integrating circuit means coupled to the outputof said AC-to-DC converter for causing load current to continue to flowduring the interval between successive coded input pulses and said peakdetector being incapable of increasing said load current during acritical circuit or component failure.
 2. A fail-safe peak detector asdefined in claim 1, wherein said first coupling means comprises atransformer having a primary and a pair of secondary windings.
 3. Afail-safe peak detector as defined in claim 1, wherein saidvoltage-to-current converter comprises an amplifying circuit having twoactive elements.
 4. A fail-safe peak detector as defined in claim 1,wherein said voltage-to-current converter comprises a class B push-pullamplifier.
 5. A fail-safe peak detector as defined in claim 1, whereinsaid AC-to-DC converter comprises a full-wave bridge rectifier.
 6. Afail-safe peak detector as defined in claim 1, wherein said secondcoupling circuit comprises a transformer having a pair of primarywindings and a secondary winding.
 7. A fail-safe peak detector asdefined in claim 1, wherein said integrating circuit means includes anelectromagnetic relay.
 8. A fail-safe peak detector as defined in claim7, wherein said electromagnetic relay includes a magnetic coil coupledto the output of said AC-to-DC converter and an electrical contactconnected to an indication circuit.
 9. A fail-safe peak detector asdefined in claim 8, wherein said magnetic coil includes a seriesinductance and resistance elements.
 10. A fail-safe peak detector asdefined in claim 9, wherein the time consTant of said series inductanceand resistance elements is substantially longer than the pulserepetition period of said coded pulsating input signal.
 11. A fail-safecurrent-type of peak detection circuit comprising a source of codedpulsating signals having a low duty cycle in comparison with the pulserepetition rate, an input transformer having a primary windingelectrically connected to said pulsating signal source and having a pairof secondary windings, a push-pull amplifier, said pair of secondarywindings electrically connected to the inputs of said push-pullamplifier, and an output transformer having a pair of primary windingsand a secondary winding, said pair of secondary windings electricallyconnected to the output of said push-pull amplifier, a rectifier havingits input terminals connected to said secondary winding and having itsoutput terminals connected to a coil of an electromagnetic relay whichremains picked up during the off periods of said coded pulsating signalsand which is incapable of being picked up by a critical circuit orcomponent failure.
 12. A fail-safe current-type of peak detectioncircuit as defined in claim 11, wherein said coded pulsating signalsource comprises a high frequency signal source having a duty cycle ofapproximately 10 percent of the time.
 13. A fail-safe current-type ofpeak detection amplifier includes a pair of semiconductive devicesoperating in class B service.
 14. A fail-safe current-type of peakdetection circuit as defined in claim 11, wherein said rectifiercomprises a full-wave bridge rectifier.
 15. A fail-safe current-type ofpeak detection circuit as defined in claim 11, said coil of saidelectromagnetic relay comprises a series conductive resistive circuithaving a time constant which is relatively large in comparison with saidpulse repetition rate.